Why does SmarterMail, when acting as an Incoming Gateway doing SMTP User Verification, give a Permanent Negative Completion Reply of "550 No such user here" response when there is a timeout rather than provide a Transient Negative Completion Reply like "450 Requested mail action not taken: mailbox currently unavailable" or "451 Requested action aborted: local error in processing"?

If the Incoming Gateway is temporarily unable to communicate with the primary SmarterMail server, due to high CPU, network latency, or a brief restart of the MailService, then it would stand to reason that not getting a response should indicate a temporary error that should result in a transient negative reply or a 4XX response so that the connecting Mail Server can try again later rather than giving it a permanent negative 5XX response that immediately results in a NDR (Non-Deliverable Response) bounce-back.

In addition, a SMTP User Verification timeout should NEVER be cached in the \SmarterMail\Service\Settings\SmartHostCaches\ as a permanent negative reply! THIS IS VERY IMPORTANT AND NEEDS TO BE CHANGED ASAP! If the Incoming Gateway does not get a response then it shouldn't be caching the lack of a response! It should only be caching verifications, not lack of verifications due to timeouts! Don't bounce all incoming emails for the next X# of minutes because of a single dropped packet or lack of ACK response!

I'm going to explain why this quickly becomes a nightmare scenario:

Fortune 500 company uses a third-party bulk mailing service, such as MailChimp, SparkPost, SendGrid, ConstantContact, etc. to send transactional emails. To avoid abuse these third-party bulk mailing services now have a One Bounce rule that if an email is rejected with a permanent negative reply then that email address can no longer be mailed to, resulting in the recipient never receiving their receipts from Paypal, or Walmart, or their doctor, hospital, or pharmacist! What's even worse is that third-party bulk mailing service also handles transactional emails for hundreds of other companies. Now those other companies aren't sending any emails to that recipient as their email address is now on a "suppression list" to prevent further abuse. Other than contacting every company that does business with that third-party bulk mailing service to request that your address be removed from their "suppression list" you simply won't be getting emails sent via that service ever again. When X number of users on the same domain get on that "suppression list" the entire domain gets added to the suppression list and no one that you host email for is getting their emails...all because the MailService.exe service was busy or restarted and it was temporarily unavaible to the Incoming Gateway for SMTP User Verification for 30 seconds!

Please, please, please, make temporary errors give transient responses, not permanent ones!